A chain transmits mechanical power from one or more crank sprockets to one or more hub sprockets on a bicycle. If the bicycle has multiple sprockets at the crank or the hub, it may also have a spring-loaded derailleur. In addition to shifting the chain between sprockets, the derailleur can maintain the tension of the chain. On single-speed and enclosed-hub bicycles, however, where derailleurs are typically absent, some other means of chain tensioning may be provided.
On some bicycles, the rear axle is mounted in a horizontally elongated rear dropout, more commonly called a horizontal rear dropout. Sliding the rear axle back and forth within the horizontal rear dropout changes the distance between the rear sprocket and the crank sprocket, thus changing the tension of the chain. The horizontal rear dropout does not ideally solve the problem of chain tensioning, however, as it can allow slippage of the axle and misalignment of the wheel. Moreover, it can be difficult to align disk brakes on bicycles that have horizontal rear dropouts, because changing the position of the rear axle within the rear dropout offsets the wheel-mounted disk with respect to the frame-mounted brake caliper.